In this study we will examine pressure-volume hysteresis of isolated lobes and lung tissue under dynamic conditions. An attempt will be made to distinguish between the tissue and surface forces involved in generating the hysteresis. Separation of the effects of these forces will be accomplished by alteration of the properties of the air-liquid interface, either increasing or eliminating the interfacial surface tension. A new technique will be used to examine a small piece of lung tissue in an effort to minimize technical problems as well as problems with airway resistance, which are substantial with liquid inflations. We will compare our experimental results with the theoretical predictions of a mathematical model which describes the behavior of the surface forces in an alveolus under dynamic conditions.